arXiv subjects

Tomo Takahashi

Publications and source records attributed to Tomo Takahashi.

157 records · Page 9Linked to original sources

arXiv2004

Candidates for Inflaton in Quiver Gauge Theory

The inflaton necessary to implement the mechanism of inflationary cosmology has natural candidates in quiver gauge theory. We discuss the dimensionless coefficients of quartic couplings and enumerate scalars which are singlet under the low-energy gauge group. The inflaton mass is generally predicted to be in the TeV region, close to 4 TeV for one specific unified model. A quartic inflaton potential, and a mutated hybrid inflation, are discussed. They can give adequate inflation and appropriate fluctuations but different spectral indices.

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arXiv2004

Correlated Isocurvature Fluctuation in Quintessence and Suppressed CMB Anisotropies at Low Multipoles

We consider cosmic microwave background (CMB) anisotropy in models with quintessence taking into account of isocurvature fluctuation in the quintessence. It is shown that, if the primordial fluctuation of the quintessence has a correlation with the adiabatic density fluctuations, CMB angular power spectrum $C_l$ at low multipoles can be suppressed without affecting $C_l$ at high multipoles. Possible scenario of generating correlated mixture of the quintessence and adiabatic fluctuations is also discussed.

astro-ph↗

arXiv2003

Robustness and Predictivity of 4 TeV Unification

The stability of the predictions of two of the standard model parameters, $α_3(M_Z)$ and $\sin^2 θ(M_Z)$, in a $M_U \sim 4$ TeV unification model is examined. It is concluded that varying the unification scale between $M_U \simeq 2.5$ TeV and $M_U \simeq 5$ TeV leaves robust all predictions within reasonable bounds. Choosing $M_U = 3.8 \pm 0.4$ TeV gives, at lowest order, accurate predictions at $M_Z$. The impact of threshold effects on unification depends on the spectrum of states beyond the standard model.

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arXiv2003

Cosmic Degeneracy with Dark Energy Equation of State

We discuss a degeneracy between the geometry of the universe and the dark energy equation of state $w_{\rm X}$ which exists in the power spectrum of the cosmic microwave background. In particular, for the case with $w_{\rm X} < -1$, this degeneracy has interesting implications to a lower bound on $w_{\rm X}$ from observations. It is also discussed how this degeneracy can be removed using current observations of type Ia supernovae.

astro-ph↗

arXiv2003

The Fate of Dark Energy

By studying the present cosmological data, particularly on CMB, SNeIA and LSS, we find that the future fate of the universe, for simple linear models of the dark energy equation-of-state, can vary between the extremes of (I) a divergence of the scale factor in as little as 7 Gyr; (II) an infinite lifetime of the universe with dark energy dominant for all future time; (III) a disappearing dark energy where the universe asymptotes as $t \to \infty$ to $a(t) \sim t^{2/3}$ {\it i.e.} matter domination. Our dreadful conclusion is that no amount of data from our past light-cone can select between these future scenarios.

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arXiv2002

Cosmic Microwave Background from Late-Decaying Scalar Condensations

We study the cosmic microwave background (CMB) anisotropy in the scenario with late-decaying scalar condensations which arise in many class of cosmological scenarios based on supersymmetric models. With such a scalar condensation ϕ, the CMB radiation we observe today originates to ϕand hence the CMB anisotropy is affected if ϕhas a primordial fluctuation. In particular, if all the components in the universe (i.e., radiation, baryon, cold dark matter, and so on) are generated from the decay product of ϕ, the dominant source of the cosmic density perturbations can be the primordial fluctuation of ϕ, not the fluctuation of the inflaton field. In this case, the constraints on the inflation models can be drastically relaxed. In other case, the baryon or the CDM may not be from the decay product of ϕand correlated mixture of the adiabatic and isocurvature fluctuations can be generated. If so, the CMB angular power spectrum may not be the same as the adiabatic result and the on-going MAP experiment may observe a deviation from the prediction of the standard inflationary scenario.

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arXiv2002

Effects of Cosmological Moduli Fields on Cosmic Microwave Background

We discuss effects of cosmological moduli fields on the cosmic microwave background (CMB). If a modulus field ϕonce dominates the universe, the CMB we observe today is from the decay of ϕand its anisotropy is affected by the primordial fluctuation in the amplitude of the modulus field. Consequently, constraints on the inflaton potential from the CMB anisotropy can be relaxed. In particular, the scale of the inflation may be significantly lowered. In addition, with the cosmological moduli fields, correlated mixture of adiabatic and isocurvature fluctuations may be generated, which results in enhanced CMB angular power spectrum at higher multipoles relative to that of lower ones. Such an enhancement can be an evidence of the cosmological moduli fields, and may be observed in future satellite experiments.

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arXiv2002

Cosmic Density Perturbations from Late-Decaying Scalar Condensations

We study the cosmic density perturbations induced from fluctuation of the amplitude of late-decaying scalar condensations (called ϕ) in the scenario where the scalar field ϕonce dominates the universe. In such a scenario, the cosmic microwave background (CMB) radiation originates to decay products of the scalar condensation and hence its anisotropy is affected by the fluctuation of ϕ. It is shown that the present cosmic density perturbations can be dominantly induced from the primordial fluctuation of ϕ, not from the fluctuation of the inflaton field. This scenario may change constraints on the source of the density perturbations, like inflation. In addition, a correlated mixture of adiabatic and isocurvature perturbations may arise in such a scenario; possible signals in the CMB power spectrum are discussed. We also show that the simplest scenario of generating the cosmic density perturbations only from the primordial fluctuation of ϕ(i.e., so-called ``curvaton'' scenario) is severely constrained by the current measurements of the CMB angular power spectrum if correlated mixture of the adiabatic and isocurvature perturbations are generated.

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arXiv2002

Isocurvature Fluctuations in Tracker Quintessence Models

We consider effects of the isocurvature perturbation in the framework of the tracker-type quintessence models. During the inflation, fluctuations in the amplitude of the quintessence field are generated, which provide isocurvature component of the cosmic density perturbation. Contrary to the conventional notion, it is shown that effects of the isocurvature fluctuation may become sizable in some cases, and in particular, the cosmic microwave background angular power spectrum may be significantly enhanced due to the effect of the isocurvature mode. Such an enhancement may be detectable in on-going and future experiments.

astro-ph↗

arXiv2001

Cosmic Microwave Background Anisotropy with Late Time Entropy Production

We discuss effects of cosmological moduli fields on the cosmic microwave background (CMB). If a modulus field ϕonce dominates the universe, the CMB we observe today is from the decay of ϕand its anisotropy is affected by the primordial fluctuation in the amplitude of the modulus field. As a result, constraints on the inflaton potential from the CMB anisotropy can be relaxed. In addition, with the cosmological moduli fields, correlated mixture of adiabatic and isocurvature fluctuations may be generated, which results in enhanced CMB angular power spectrum at higher multipoles relative to that of lower ones. Such an enhancement can be an evidence of late time entropy production due to the cosmological moduli fields, and may be observed at on-going and future experiments.

hep-ph↗

arXiv2001

Cosmic Microwave Background Anisotropy in models with Quintessence

We study the Cosmic Microwave Background (CMB) anisotropies produced by cosine-type quintessence models. In our analysis, effects of the adiabatic and isocurvature fluctuations are both taken into account. For purely adiabatic fluctuations with scale invariant spectrum, we obtain a stringent constraint on the model parameters using the CMB data from COBE, BOOMERanG and MAXIMA. Furthermore, it is shown that isocurvature fluctuations have significant effects on the CMB angular power spectrum at low multipoles in some parameter space, which may be detectable in future satellite experiments. Such a signal may be used to test the cosine-type quintessence models.

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arXiv2001

Cosmic Microwave Background Anisotropy with Cosine-Type Quintessence

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arXiv2001

Constraints on Natural Inflation from Cosmic Microwave Background

We study constraints on the natural inflation model from the cosmic microwave background radiation (CMBR). Inflaton ϕfor the natural inflation has a potential of the form V=Λ^4[1-\cos(ϕ/\sqrt{2}f_ϕ)], which is parametrized by two parameters f_ϕand Λ. Various cosmological quantities, like the primordial curvature perturbation and the CMBR anisotropy, are determined as functions of these two parameters. Using recent observations of the CMBR anisotropy by BOOMERANG and MAXIMA (as well as those from COBE), constraints on the parameters f_ϕand Λare derived. The model with f_ϕlower than 8.5\times 10^{18} (5.4\times 10^{18}, 4.5\times 10^{18}) GeV predicts a power spectrum with index n_{\rm S} smaller than 0.95 (0.9, 0.85) which suppresses the CMBR anisotropy for smaller angular scale. With such a small n_{\rm S}, height of the second acoustic peak can become significantly lower than the case of the scale-invariant Harrison-Zeldovich spectrum.

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